1. Field of the Invention
The present invention relates to an optical encoder, particularly to an optical encoder enabling to be fabricated at low cost by dispensing with high assembling accuracy by adopting a telecentric optical system.
2. Description of the Related Art
As is well known, an optical encoder of this kind includes a graduation plate inscribed with a graduation line along a predetermined moving direction and an optical reader for optically reading the graduation line on the graduation plate. When the graduation plate and the optical reader are moved relative to each other in cooperation with movement of a detection object, a corresponding electric signal is outputted from the optical reader.
A mode of a graduation plate differs between a case of a rotary encoder and a case of a linear encoder. In the case of the rotary encoder, a rotating disk is adopted as a graduation plate and the graduation plate and an optical reader are supported movably relative to each other in a circumferential direction. In the case of the linear encoder, a slender long plate (referred to as “scale” or the like) is used as a graduation plate and the graduation plate and the optical reader are supported movably relative to each other in a linear direction.
Significance of a graduation line inscribed on a graduation plate differs between a case of an increment type encoder and a case of an absolute type encoder. In the case of the increment type encoder, graduation lines are inscribed on a graduation plate over one or two or more of rows at equal pitch. When the graduation plate and the optical reader are moved relative to each other, single phase or multi-phase pulse rows of a number in correspondence with a moving distance or a rotating angle are outputted from the optical reader. In a general example, the two phase pulse rows having a phase difference of 90° are outputted from the optical reader. In the case of the absolute type encoder, respective rows on the graduation plate are inscribed with graduation lines in correspondence with multi-bit codes indicating absolute positions thereof. When the graduation plate and the optical reader are moved relative to each other, the multi-bit codes directly indicating a moving position or a rotating angle are outputted from the optical reader. In a general example, gray codes are adopted as the multi-bit codes inscribed on the graduation plate.
A method of inscribing the graduation line on the graduation plate differs by whether a style of the optical reader is a transmission type or a reflection type. When the style of the optical reader is the transmission type, the graduation line is inscribed on the graduation plate by penetrating a slit through a light blocking plate of a metal or the like by etching, or making a linear transparent portion emerge on a glass plate having a light blocking film by carrying out etching treatment. When the style of the optical reader is the reflection type, the graduation line is inscribed on the graduation plate by forming a mirror face reflecting portion by vapor-depositing a metal-made film on a glass plate and thereafter making a linear transparent portion emerge by carrying out etching treatment or carrying out printing treatment having high contrast on a surface of a material plate. In the case of use requesting high accurate positional detection, a pitch of inscribing the graduation line falls in a range of several 10 μm through several 100 μm.
The optical reader includes a light projecting optical system for irradiating light from a light projecting source to the graduation plate and a light receiving optical system for focusing the light from the graduation plate to a light receiving portion. In the case of the transmission type optical reader, the light projecting optical system including the light projecting source and the light receiving optical system including the light receiving portion are arranged opposedly to each other while interposing the graduation plate. In the case of the reflection type optical reader, the light projecting optical system including the light projecting source and the light receiving optical system including the light receiving portion are summarizingly arranged on one side of the graduation plate.
Further, a detecting system utilizing diffraction interference is also adopted in a highly fine detection region having a constant correlation between the pitch of the correlation line on the graduation plate and light wavelength of a semiconductor laser constituting the light projecting source.
According to the optical encoder of this kind, in order to enable highly accurate positional measurement, in a procedure of moving the graduation plate and the optical reader relative to each other, a clear image of the graduation line needs to continue focusing on a light receiving face via the light receiving optical system.
However, as the light receiving optical system of the transmission type optical encoder, a lens having comparatively shallow object depth is used and therefore, when the distance between the graduation plate and the optical reader is varied by applying excessively large shaft load or fluctuating the rotating disk in the rotary encoder, immediately, clearness or size of the graduation line image focused on the light receiving face is also varied, a noise component emerges in an output signal of the optical reader, as a result, detection accuracy is deteriorated regardless of the absolute type or the increment type.
Further, in the case of the increment type encoder adopting the transmission type optical reader, in order to exclude influence or stray light from a contiguous slit (graduation line) on the graduation plate, a clearance between the graduation plate and the light blocking plate needs to be controlled strictly, high assembling accuracy is needed and an increase in cost is brought about.
FIG. 21 is a view showing a structure of a transmission type rotary encoder used in a related art and FIG. 22 enlarges an essential portion indicated by a one-dotted chain line circle Y in FIG. 21 for convenience of explanation. In FIGS. 21 and 22, numeral 802 designates a light emitting diode (LED), numeral 801 designates a case of the light emitting diode (LED), numeral 803 designates a rotating disk, numeral 804 designates a light blocking plate, numeral 805 designates a light receiving element, numeral 806 designates a board for mounting the light receiving element and numeral 807 designates a case of the rotary encoder, respectively.
As is shown by an enlarged view of FIG. 22, a clearance C between the graduation plate (rotating disk 803) and the light blocking plate 804 is extremely narrow and therefore, in addition to the above-described problem, in the case of the rotary encoder, when the rotating disk 803 is inclined by applying excessively large shaft load, the rotating disk 803 is brought into abrasive contact with the light receiving portion (light receiving plate 804 and light receiving element 805) and destruction is brought about.
Meanwhile, as an encoder adopting a reflection type optical reader, there is a diffraction interference reflecting type linear encoder, however, from the detection principle, the pitch of the graduation line is obliged to be small in relation to wavelength of laser and the encoder cannot deal with a variety of pitches (resolution). In addition thereto, a special scale is needed, which is devoid of general purpose performance.
Meanwhile, in the case of an encoder adopting a reflection type optical reader having a light projecting optical axis and a light receiving optical axis independently from each other, the two optical axes are inclined symmetrically relative to a normal line of the graduation plate and therefore, a variation in clearness or size of the graduation line image by fluctuating or inclining the graduation plate is considerable and the encoder cannot be reduced to practice for use of highly accurate detection.